JP3601182B2

JP3601182B2 - Optical head device and manufacturing method thereof

Info

Publication number: JP3601182B2
Application number: JP10032996A
Authority: JP
Inventors: 譲田辺; 弘昌佐藤; 弘樹保高
Original assignee: Asahi Glass Co Ltd
Current assignee: AGC Inc
Priority date: 1996-04-22
Filing date: 1996-04-22
Publication date: 2004-12-15
Anticipated expiration: 2016-04-22
Also published as: JPH09288844A

Description

【０００１】
【発明の属する技術分野】
本発明は、光磁気メディアの情報を読み取るための光ヘッド装置に関する。
【０００２】
【従来の技術】
従来、ミニディスク等の光磁気ディスクの光学的情報を読み取る光ヘッド装置には光偏光素子を用いている。その光ヘッド装置としては、光磁気ディスクの記録面から反射された信号光を検出部へ導光（ビームスプリット）する光学部品としてプリズム式ビームスプリッタを用い、その出射光を光偏光素子を通して複数の回折光に分けて検出部で光を検出している。
【０００３】
この光偏光素子としては、ＬｉＮｂＯ_３等の複屈折単結晶を加工した２つの異なる結晶方向をもった結晶片を接合せしめたものが使用されていた。しかし、これらの単結晶の光偏光素子は、単結晶を用いているため、また、その加工が複雑なため高価なものであり、大量生産が困難であった。
【０００４】
【発明が解決しようとする課題】
本発明は前述の問題を解決すべくなされたものであり、大量生産に適し、高い信頼性を有する光ヘッド装置の提供を目的とする。
【０００５】
【課題を解決するための手段】
【０００６】
本発明は、光源からの光を光記録媒体に照射することにより、情報を読み取り及び／又は情報を書き込む光ヘッド装置であって、光源と光記録媒体との間に光源からの光を透過し光記録媒体からの反射光が光源に戻らないように光の光路を変えるビームスプリッタを配置し、この光路を変えられた反射光を回折素子を通過させて、複数の回折光に分けて検出部で光を検出する光ヘッド装置において、回折素子の２枚の透明基板の各々に周期的に電極が形成されていて、一方の透明基板の電極の幅が他方の透明基板の電極の幅より狭く、２つの電極が非対称に配設されていて、それらの電極に電界を印加しつつ液晶を硬化させることにより、高分子液晶の配向方向の周期的にな変化が形成されていることを特徴とする光ヘッド装置を提供する。また、その周期的に形成された電極の幅が、電極ピッチの２０〜８０％である光ヘッド装置を提供する。
【０００７】
また、光学異方性回折素子の２枚の透明基板の少なくとも一方の基板の電極はパターニングしてあり、電極に電界を印加した際に、電極が対向していない部分では液晶は自然な配列をし、電極が対向している部分では強制的な配向をし、この状態で液晶を硬化させて、高分子液晶の配向方向が周期的に変化している光学異方性回折素子を形成したことを特徴とする上記光ヘッド装置を提供する。
【０００８】
また、光学異方性回折素子の上に周期的に遮光パターンを形成したマスクを配置して、２枚の透明基板の電極全面に電界を印加しつつ又は電界を印加せずに、光露光により光が当たった部分のみ液晶を硬化させ、次いで電界の印加状態を変えて残りの部分を硬化させて、高分子液晶の配向方向が周期的に変化している光学異方性回折素子を形成したことを特徴とする上記光ヘッド装置を提供する。
【０００９】
すなわち、高分子液晶の配向に周期的な構造を持たせることにより、光学異方性回折格子を形成し、光偏光素子を実現する。
【００１０】
【発明の実施の形態】
図１は、本発明の光ヘッド装置の模式図である。図１において、１は光源、２はビームスプリッタ、３は集光レンズ、４は光記録媒体、５は光学異方性回折格子、６Ａ、６Ｂ、６Ｃは検出部である。なお、この図では分かりやすくするために、有効に利用される光のみを示している。
【００１１】
光源１からでた光は、ビームスプリッタ２に入射するが、ある割合でそのまま透過し、集光レンズ３で集光されて光記録媒体４に到達する。この光記録媒体４で反射された光は、ビームスプリッタ２によりある割合で光路を変えられ光学異方性回折格子５に入射する。この光学異方性回折格子５では、その入射光の偏光方向に応じて光が回折するように回折格子が形成されていて、光が分けられて検出部６Ａ、６Ｂ、６Ｃで検出される。
【００１２】
ビームスプリッタ２は、図示のものはプリズムを組み合わせたものとされ、光源から光のある割合が直進し、光記録媒体からの反射光のある割合がその光路を９０°曲げられ、再度９０°曲げられて、光源とほぼ同じ位置にある検出部で光を検出するようにされている。
【００１３】
なお、本発明におけるビームスプリッタは、光源からの光のある割合が透過し光記録媒体からの反射光のある割合が検出部に到達するようにされていればよい。たとえば、光源からの光が９０°曲げられて光記録媒体に向かうようにされ、光記録媒体からの反射光が直進するようにされてもよい。また、この曲げられる角度も９０°には限られない。なお、光ヘッド装置の小型化という面からは図１のような構成が有利である。
【００１４】
本発明では、光源１から出た光は光記録媒体４に到達する前には光学異方性回折格子５を通過しない。このため、反射光はビームスプリッタを出て光学異方性回折格子を通過後に、再度鏡やプリズムで光路を変更されてもよい。
【００１５】
また、検出部は図１では３個設けられているが、これの数や位置はその光ヘッドで要求される数の検出部を必要な位置に配置すればよい。
【００１６】
本発明では、この光学異方性回折格子５が２枚の透明基板間に光学異方性を有する高分子液晶を挟持したものであり、その高分子液晶の配向方向が周期的に変化している光学異方性回折素子とされる。
【００１７】
図２は、本発明で用いられる光学異方性回折素子の断面図である。図２において、１１Ａ、１１Ｂはガラス、プラスチック等の透明基板、１２Ａ、１２ＢはＩｎ_２Ｏ_３ −ＳｎＯ_２（ＩＴＯ）、ＳｎＯ_２等の電極、１３Ａ、１３Ｂはポリイミド、ポリアミド、ＳｉＯ等の配向膜、１４は高分子液晶、１５はその水平配向部分、１６はその垂直配向部分を示す。
【００１８】
本発明の光学異方性回折素子では、その製造のために２枚の透明基板に電極を設ける。特に、その少なくとも一方の透明基板の高分子液晶に接する側の面に、周期的な電極を設けることが好ましい。具体的には、一方が周期的な電極で他方がべた電極、又は両方を周期的な電極とすることが好ましい。
【００１９】
図３は、この周期的な電極の例の平面図である。電極のある部分２２と電極のない部分２１とが交互に周期的に並んで配置されている。図２のＷ_１は電極の幅、Ｗ_２は電極のない部分の幅、図２及び図３のＷ_３は電極のピッチである。この図２及び図３の例では、電極は２枚の基板とも同じ幅で両方ともストライプ状の周期的なパターンとされている。
【００２０】
このような２枚の透明基板で未重合の液晶材料（液晶性モノマー）を挟持し、前記電極に電界を印加した状態で重合させることによって、周期的な配向構造を持った高分子液晶による光学異方性回折格子を形成する製造方法が好ましい。これにより、容易に配向状態が周期的に変化している高分子液晶が形成でき、光学異方性回折格子を生産性良く大量生産可能になる。
【００２１】
このとき、未重合の液晶材料は、使用する液晶が正の誘電異方性を有する場合には、配向膜は水平配向するものを用いる。通常はラビング等により特定の方向に配向処理をしておく。このようにすると、電界を印加された部分は液晶分子が電界に平行で透明基板に垂直に配向する。電界を印加しない部分は、透明基板に平行で配向膜の配向処理方向に沿って配向する。
【００２２】
図４は、この電界を印加している状態での液晶の配向状態を示す模式図である。この例は、正の誘電異方性のネマチック液晶を用いた例を示し、上側の基板の電極３１Ａ、３１Ｂ、３１Ｃには全て同じ極性の電界が印加され、同様に下側の基板の電極３２Ａ、３２Ｂ、３２Ｃには全て同じ極性の電界が印加されている。このため、電極が対向している部分３５では、液晶は垂直配向する。一方、電極が対向していない部分３６では、液晶はほぼ配向膜の配向処理方向に従って配向する。
【００２３】
かなり高い電界を印加した場合には、この電極が対向していない部分３６でも液晶分子が立ち上がり始めるが、本発明のような回折格子では特性が悪くなるので、通常は電極が対向している部分３３、３５、３７でほぼ充分に液晶が垂直配向して、電極が対向していない部分３４、３６では、ほぼ水平配向して両部分での屈折率の差が大きくなるように電圧やその周波数（直流か交流かも含め）を定めればよい。
【００２４】
使用する液晶材料が負の誘電異方性を有する場合には、配向膜は垂直配向するものを用いる。この場合にも、通常はラビング等により特定の方向に配向処理をしておく。このようにすると、電界を印加しない部分は透明基板に垂直に、電界を印加した部分は配向処理方向に沿って透明基板に平行に配向する。
【００２５】
配向膜の配向能力の違いを利用して、フォトリソグラフィ法とラビング法の組合せによって、垂直配向領域と水平配向領域の周期的パターンを形成することもできる。また、電界を交互にかけることによって、電界分布を改善できる。この場合、配向膜は省略できる。
【００２６】
図５は、この電界を印加している状態での液晶の配向状態を示す模式図である。この例も正の誘電異方性のネマチック液晶を用いた例を示し、上側の基板の電極４１Ａ、４１Ｂ、４１Ｃには１個おきに逆の極性の電界が印加され、同様に下側の基板の電極４２Ａ、４２Ｂ、４２Ｃには同様に１個おきに逆の極性の電界が印加されている。
【００２７】
このため、電極が対向している部分４３、４５、４７では、図４の例と同様に液晶は垂直配向する。一方、電極が対向していない部分４４、４６では、液晶は隣接電極間の電界の影響も受け、その方向に配向する。このため、配向膜を設けていなくても特定の方向に配向できる。
【００２８】
上記の方法により、液晶材料の配向に分布を付与した状態のまま、熱、紫外線等により全体を高分子化することにより、配向の分布を固定したまま固化させうる。
【００２９】
本発明の高分子液晶とは、未重合の液晶材料が液晶性を示せばよく、そのような未重合の液晶材料（液晶性モノマー）から生成したポリマーであって、ここでは０．０２以上の屈折率異方性を示すものであればよい。この屈折率異方性は、大きいものが好ましく、０．１以上となるポリマーが好ましい。したがって高分子液晶自身が液晶性を示す必要はない。
【００３０】
高分子液晶は液晶性モノマーを光又は熱によって重合して製造することが好ましい。特に紫外光又は可視光で重合しうる液晶モノマーは、フォトリソプロセスによってオンサイトで（基板上で直接）高分子液晶を製造でき、好ましい。
【００３１】
液晶性モノマーとは室温又は光重合時の温度において液晶性を示すモノマーをいう。液晶性とはネマチック、スメクチック、コレステリックなど公知の液晶相を示すことをいうが、螺旋のピッチが短いコレステリックの場合は本発明にそぐわず不適である。
【００３２】
液晶性モノマーとしては、アクリル酸又はメタクリル酸等のエステル類中から選ぶのが好ましい。エステルを形成するアルコール残基にフェニル基が１個以上、特には２個又は３個含まれていることが好ましい。さらにエステルを形成するアルコール残基にシクロヘキシル基が１個含まれていてもよい。液晶性モノマーはその液晶として存在できる温度範囲を広げるために、２成分以上混合して用いうる。
【００３３】
本発明の態様では、前記２枚の透明基板には各々電極が設けられ、２つの電極の一方は周期的に形成されていることが好ましい。このような構成により、周期的に形成された分割電極に相当する部分と、分割電極が形成されていない部分とで、電界印加時の液晶材料の配向状態を異なるようにでき、光学異方性回折格子を電界により容易に形成できる。
【００３４】
前記の図の例のように、２つの電極の両方が周期的に形成されてなり、前記２枚の透明基板間において前記２つの電極を対称とすると、この光学異方性回折格子を出射した±１次回折光を両方ともほぼ同じ回折効率となしうる。
【００３５】
また、本発明の別の態様では、２つの電極の両方が周期的に形成されてなり、前記２枚の透明基板間において前記２つの電極が非対称とされる。具体的には、たとえば、図４の例で見た場合、電極３１Ａに対して電極３２Ａがその右側だけ電極３２Ｂ側に出ており、電極３２Ａの幅が電極３１Ａの幅よりも広くされる。
【００３６】
こうすることにより、高分子液晶セルが形成された状態において、２枚の透明基板間の中心に位置し２枚の透明基板に平行な中心面に関して、非対称になる。このような構成により、２つの電極の各々の分割電極は、その位置及び大きさが異なる状態で対面することになり、上下一対の分割電極でみた場合、高分子液晶の分割電極による配向部を左右非対称となしうる。したがって、±１次回折光のうちいずれかの回折効率が高い光学異方性回折格子を電界により容易に形成できる。
【００３７】
本発明では、前記したように少なくとも一方の透明基板の液晶と接する側の面に配向膜が設けられ、少なくとも一方の配向膜が周期的に配向力の異なる配向膜を含んでいるようにもなしうる。このような配向力の周期的に変化している分割配向膜を用いても、高分子液晶の配向状態に分布を付与できる。さらには、周期の方向における高分子液晶の配向状態を左右非対称ともなしうる。したがって、±１次回折光のうちいずれかの回折効率が部分的に高い光学異方性回折格子を容易に形成できる。
【００３８】
さらに、２枚の透明基板の両方の配向膜を分割配向膜ともなしうる。このため、高分子液晶セルが形成された状態において、２枚の透明基板間の中心に位置し２枚の透明基板に平行な中心面に関して、配向状態を非対称となしうる。このような構成により、２つの配向膜の各々の分割配向膜は、上下１対の分割配向膜でみた場合、高分子液晶の分割配向膜による配向部を左右非対称となしうる。したがって、±１次回折光のうちいずれかの回折効率が高い光学異方性回折格子を配向膜により容易に形成できる。
【００３９】
また、本発明のさらに別の態様では、光学異方性回折素子の上に周期的に遮光パターンを形成したマスクを配置して、２枚の透明基板の電極全面に電界を印加しつつ、又は電界を印加せずに光露光により光が当たった部分のみ液晶を硬化させ、次いで電界の印加状態を変えて残りの部分を硬化させて、高分子液晶の配向方向が周期的に変化している光学異方性回折素子を形成することもできる。
【００４０】
さらには、このようなマスクを用いて、基板の外側に電極を配置して電界を印加したり、基板の外側から磁界を印加したりして周期的な配向を形成するようにしてもよい。
【００４１】
本発明の光ヘッド装置は、光学的には以下のように機能する。なお、光学異方性回折格子は、紫外線硬化性の未重合の正の誘電異方性のネマチック液晶を用いて、紫外線硬化を行い図２のように構成したものを用いたとして説明する。
【００４２】
光源から出射して光記録媒体で反射して、ビームスプリッタで光路を変更され、光学異方性回折格子に入射する。このとき、Ｓ偏光（図２の紙面に垂直な偏光成分）に対しては、光学異方性回折格子は水平配向部分と垂直配向部分とが光学的に一様、すなわち両方の部分とも常光屈折率になる。このため、光が何の変化も受けず透過する。
【００４３】
一方、Ｐ偏光（図２の紙面に平行な偏光成分）は、垂直配向部分と水平配向部分との屈折率が異なることになる。すなわち、垂直配向部分は常光屈折率、水平配向部分は異常光屈折率となる。このため、回折格子として認識され、理論上は＋１次光として〜４０％、−１次光として４０％の回折光が得られ、透過光は理論的にはゼロにすることができる。残りは高次光となる。
【００４４】
そのため、たとえば透過光量、±１次回折光量の相対強度比を測定することによって、光記録媒体からの反射光の偏光状態及びその変化を読み取りうる。
【００４５】
この例の光学異方性回折格子は、電極の幅を２枚の基板で同じにしたが、２枚の透明基板に各々設けられた２つの周期的な電極を、その位置及び／又は大きさを非対称にすることにより、電極部に相当し電界によって特定方向に配向された高分子液晶を左右非対称に形成できる。したがって、±１次回折光のいずれかの回折効率が高い光学異方性回折格子となしうる。
【００４６】
また、この電極の幅Ｗ₁ は電極ピッチのほぼ半分程度にすればよいが、必要に応じて２０〜８０％程度にされる。また、電極のピッチＷ₃ は５〜５０μｍ程度の範囲で適宜最適化して用いればよい。
【００４７】
【実施例】
[ 例１ ] は参考例、 [ 例２ ] は実施例である。
［例１］
厚さ１ｍｍ、１２０ｍｍ×１２０ｍｍ角のガラス基板の１表面に、ＩＴＯのべたの透明電極を形成し、フォトリソグラフィ法とドライエッチング法により、ＩＴＯの透明電極を幅Ｗ₁ ＝１０μｍ、ピッチＷ₂ ＝２０μｍのストライプ状にパターニングした。なお、反対の面には反射防止膜を蒸着法により形成した。
【００４８】
その後、スピンコート法により約６０ｎｍ程度のポリイミド膜を形成し、ラビングにより水平配向処理を行った。このようにして形成した２枚のガラス基板の透明電極を形成した面が相対向するように配置し、周辺をシール材で封止して、セルギャップが５μｍの注入口を設けた空セルを形成した。
【００４９】
その空セルに、ｐ−［４−ω−アクリロイルオキシアルキルオキシ）フェニルカルボニルオキシ］ベンゾニトリルとｐ−（４−ω−アクリロイルオキシアルキルオキシ）安息香酸−ｐ’−ｎ−アルキルオキシフェニルエステルを主成分とする液状の液晶モノマーに光重合開始材としてベンゾインイソプロピルエーテルを１％添加した組成物（正の誘電異方性のネマチック液晶として動作）を注入した。
【００５０】
その後、２枚の基板の電極間に５Ｖの電界を印加し、電極が対向している部分の液晶組成物を垂直に配向せしめた。その後波長３６０ｎｍの紫外線を全体に照射し、上記配向を保持したまま、全体を重合せしめ、硬化することによって、配向を固定する。
【００５１】
これにより形成された光学異方性回折格子はＳ偏光（図２の紙面に垂直に偏光した光）に対しては、電界印加部で屈折率１．５２（常光屈折率）、非印加部で屈折率１．５３（常光屈折率）、Ｐ偏光（図２の紙面に平行方向に偏光した光）に対しては、電界印加部で屈折率１．５４（常光屈折率）、非印加部で屈折率１．６６（異常光屈折率）が得られ、屈折率差としては０．１２程度が得られた。
【００５２】
この光学異方性回折格子を用いた図１の構成の光ヘッド装置で、光源として波長６７８ｎｍのレーザ光源を用いた場合、Ｓ偏光波入射光に対する光透過率は８５％、光回折効率は±１次とも０．５％であった。Ｐ偏光波入射光に対する光透過率は３．４％で、光回折率は＋１次が３４％、−１次が３２％であった。
【００５３】
［例２］
例１の透明電極の構成を以下のように変えた以外は例１と同様にして回折素子を作製した。
【００５４】
一方のガラス基板の透明電極は幅Ｗ_１＝１０μｍ、ピッチＷ_２＝２０μｍのストライプ状にパターニングした。他方のガラス基板の透明電極は幅Ｗ_１＝５μｍ、ピッチＷ_２＝２０μｍのストライプ状にパターニングした。このような基板を用いて空セルを形成した他は、例１と同様にして光学異方性回折格子を形成し、光ヘッド装置を形成した。この光ヘッド装置は、＋１次回折光と−１次回折光で回折光の効率に差が生じた。
【００５５】
【発明の効果】
本発明により、大量生産可能で、高い光の利用効率と高い信頼性を有する光ヘッド装置を容易に得ることができる。また、液晶材料を適宜選択することにより、所望の屈折率を種々自由に設定でき、配向の異なる部分の形状やピッチを自由に選択できるので、所望の特性、配置の光ヘッド装置を製造しやすい。
【００５６】
さらに、液晶の配向状態を２枚の透明基板間で非対称にすることにより、非対称な光学異方性回折格子も容易に形成でき、±１次回折光のうちいずれかの回折効率が高い回折素子を容易に作製できる。
【００５７】
本発明は、本発明の効果を損しない範囲内で種々の応用ができる。
【図面の簡単な説明】
【図１】本発明の光ヘッド装置の模式図。
【図２】本発明の光ヘッド装置に用いる光学異方性回折素子の断面図。
【図３】本発明に用いる光学異方性回折素子の周期的な電極の例の平面図。
【図４】本発明に用いる光学異方性回折素子の電界を印加している状態での液晶の配向状態を示す模式図。
【図５】本発明に用いる光学異方性回折素子の電界を印加している状態での液晶の配向状態を示す模式図。
【符号の説明】
１：光源
２：ビームスプリッタ
３：集光レンズ
４：光記録媒体
５：光学異方性回折格子
６Ａ、６Ｂ、６Ｃ：検出部[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical head device for reading information on a magneto-optical medium.
[0002]
[Prior art]
Conventionally, using a light polarizing element in the optical head device which takes read optical information of the magneto-optical disc such as a mini disc. As the optical head device , a prism type beam splitter is used as an optical component for guiding (beam splitting) the signal light reflected from the recording surface of the magneto-optical disk to the detection unit, and the emitted light is transmitted to a plurality of light polarizing elements. The light is detected by the detection unit while being divided into diffracted light.
[0003]
As this light polarizing element, a device obtained by processing a birefringent single crystal such as LiNbO ₃ and joining crystal pieces having two different crystal directions has been used. However, these single-crystal optical polarization elements are expensive because of the use of single crystals and their complicated processing, making mass production difficult.
[0004]
[Problems to be solved by the invention]
The present invention has been made to solve the above-described problem, and has as its object to provide an optical head device suitable for mass production and having high reliability.
[0005]
[Means for Solving the Problems ]
[0006]
The present invention relates to an optical head device for reading information and / or writing information by irradiating light from a light source to an optical recording medium, wherein the light from the light source is transmitted between the light source and the optical recording medium. A beam splitter that changes the optical path of the light so that the reflected light from the optical recording medium does not return to the light source is arranged, and the reflected light whose path has been changed passes through the diffraction element, and is divided into a plurality of diffracted lights to be detected by the detection unit. In the optical head device for detecting light with the above, electrodes are periodically formed on each of the two transparent substrates of the diffraction element, and the width of the electrode of one transparent substrate is smaller than the width of the electrode of the other transparent substrate. The two electrodes are arranged asymmetrically, and the liquid crystal is cured while applying an electric field to those electrodes, so that a periodic change in the orientation direction of the polymer liquid crystal is formed. An optical head device is provided. Further, the present invention provides an optical head device in which the width of the periodically formed electrodes is 20 to 80% of the electrode pitch.
[0007]
In addition, the electrodes of at least one of the two transparent substrates of the optically anisotropic diffraction element are patterned, and when an electric field is applied to the electrodes, the liquid crystal has a natural arrangement in a portion where the electrodes do not face each other. Then, forced alignment was performed in the part where the electrodes faced, and the liquid crystal was cured in this state to form an optically anisotropic diffraction element in which the alignment direction of the polymer liquid crystal changed periodically. The above-mentioned optical head device is provided.
[0008]
In addition, a mask in which a light-shielding pattern is periodically formed is arranged on the optically anisotropic diffraction element, and the light exposure is performed by applying light with or without applying an electric field to the entire surface of the electrodes of the two transparent substrates. The liquid crystal was hardened only in the portion where light was applied, and then the remaining portion was hardened by changing the applied state of the electric field, thereby forming an optically anisotropic diffraction element in which the orientation direction of the polymer liquid crystal was periodically changed. The above-mentioned optical head device is provided.
[0009]
That is, by giving a periodic structure to the orientation of the polymer liquid crystal, an optically anisotropic diffraction grating is formed, and an optical polarization element is realized.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a schematic view of an optical head device according to the present invention. In FIG. 1, 1 is a light source, 2 is a beam splitter, 3 is a condenser lens, 4 is an optical recording medium, 5 is an optically anisotropic diffraction grating, and 6A, 6B, and 6C are detection units. In this figure, only light that is effectively used is shown for simplicity.
[0011]
The light emitted from the light source 1 is incident on the beam splitter 2, but is transmitted as it is at a certain rate, is condensed by the condenser lens 3, and reaches the optical recording medium 4. The light reflected by the optical recording medium 4 has its optical path changed at a certain rate by the beam splitter 2 and is incident on the optically anisotropic diffraction grating 5. In the optically anisotropic diffraction grating 5, a diffraction grating is formed so that light is diffracted in accordance with the polarization direction of the incident light, and the light is divided and detected by the detection units 6A, 6B, and 6C.
[0012]
The beam splitter 2 is a combination of prisms in the figure, and a certain percentage of light from the light source goes straight, and a certain percentage of reflected light from the optical recording medium bends its optical path by 90 ° and bends again by 90 °. Then, light is detected by a detection unit located at substantially the same position as the light source.
[0013]
The beam splitter in the present invention may be configured so that a certain ratio of light from the light source is transmitted and a certain ratio of light reflected from the optical recording medium reaches the detection unit. For example, the light from the light source may be bent by 90 degrees toward the optical recording medium, and the reflected light from the optical recording medium may travel straight. Further, the angle at which this is bent is not limited to 90 °. The configuration shown in FIG. 1 is advantageous from the viewpoint of miniaturization of the optical head device.
[0014]
In the present invention, the light emitted from the light source 1 does not pass through the optically anisotropic diffraction grating 5 before reaching the optical recording medium 4. For this reason, after the reflected light exits the beam splitter and passes through the optically anisotropic diffraction grating, the optical path may be changed again by a mirror or a prism.
[0015]
Although three detection units are provided in FIG. 1, the number and position of the detection units may be the number of detection units required by the optical head and may be arranged at necessary positions.
[0016]
In the present invention, the optically anisotropic diffraction grating 5 has a polymer liquid crystal having optical anisotropy sandwiched between two transparent substrates, and the orientation direction of the polymer liquid crystal changes periodically. Optical anisotropic diffraction element.
[0017]
FIG. 2 is a cross-sectional view of the optically anisotropic diffraction element used in the present invention. In FIG. 2, 11A and 11B are transparent substrates such as glass and plastic, 12A and 12B are electrodes such as In ₂ O ₃ —SnO ₂ (ITO) and SnO ₂ , and 13A and 13B are alignment films such as polyimide, polyamide, and SiO. , 14 are polymer liquid crystals, 15 is its horizontal alignment part, and 16 is its vertical alignment part.
[0018]
In the optical anisotropic diffraction element of the present invention, Ru is provided an electrode on the two transparent substrates for its preparation. In particular, it is preferable to provide a periodic electrode on at least one of the transparent substrates on the side in contact with the polymer liquid crystal. Specifically, it is preferable that one is a periodic electrode and the other is a solid electrode, or both are periodic electrodes.
[0019]
FIG. 3 is a plan view of an example of this periodic electrode. Portions 22 with electrodes and portions 21 without electrodes are alternately and periodically arranged. W ₁ in FIG. 2 the width of the electrode, _{W 2} is the width of the no electrode portion, W ₃ in FIG. 2 and FIG. 3 is the pitch of the electrodes. In the examples shown in FIGS. 2 and 3, the electrodes have the same width on both of the two substrates and both have a stripe-shaped periodic pattern.
[0020]
An unpolymerized liquid crystal material (liquid crystalline monomer) is sandwiched between such two transparent substrates, and is polymerized in a state where an electric field is applied to the electrodes. A manufacturing method for forming an anisotropic diffraction grating is preferred. This makes it possible to easily form a polymer liquid crystal in which the orientation state changes periodically, and to mass-produce an optically anisotropic diffraction grating with good productivity.
[0021]
At this time, as the unpolymerized liquid crystal material, when the liquid crystal used has a positive dielectric anisotropy, an alignment film that is horizontally aligned is used. Usually, orientation treatment is performed in a specific direction by rubbing or the like. In this way, the liquid crystal molecules are aligned parallel to the electric field and perpendicular to the transparent substrate in the portion where the electric field is applied. The portion to which no electric field is applied is oriented parallel to the transparent substrate and along the orientation processing direction of the orientation film.
[0022]
FIG. 4 is a schematic diagram showing an alignment state of the liquid crystal when the electric field is applied. In this example, a nematic liquid crystal having a positive dielectric anisotropy is used. An electric field of the same polarity is applied to the electrodes 31A, 31B, and 31C of the upper substrate, and the electrodes 32A of the lower substrate are similarly applied. , 32B, and 32C are all applied with an electric field of the same polarity. Therefore, the liquid crystal is vertically aligned in the portion 35 where the electrodes face each other. On the other hand, in the portion 36 where the electrodes do not face each other, the liquid crystal is aligned substantially according to the alignment processing direction of the alignment film.
[0023]
When a considerably high electric field is applied, the liquid crystal molecules start to rise even in the portion 36 where the electrode is not opposed. However, the characteristics are deteriorated in the diffraction grating as in the present invention. In the portions 34 and 36 where the liquid crystal is almost vertically aligned at 33, 35 and 37 and the electrodes 34 and 36 where the electrodes do not face each other, the voltage and its frequency are so adjusted that the difference in the refractive index between the two portions is large. (Including DC or AC).
[0024]
When the liquid crystal material to be used has a negative dielectric anisotropy, an alignment film that is vertically aligned is used. Also in this case, the alignment treatment is usually performed in a specific direction by rubbing or the like. In this way, the portion to which no electric field is applied is oriented perpendicular to the transparent substrate, and the portion to which the electric field is applied is oriented parallel to the transparent substrate along the orientation direction.
[0025]
A periodic pattern of a vertical alignment region and a horizontal alignment region can be formed by a combination of a photolithography method and a rubbing method using a difference in alignment ability of an alignment film. Further, the electric field distribution can be improved by alternately applying the electric field. In this case, the alignment film can be omitted.
[0026]
FIG. 5 is a schematic diagram showing an alignment state of the liquid crystal when the electric field is applied. This example also shows an example in which a nematic liquid crystal having a positive dielectric anisotropy is used. An electric field of opposite polarity is applied to every other electrode 41A, 41B, 41C of the upper substrate. Similarly, an electric field of opposite polarity is applied to every other electrode 42A, 42B, 42C.
[0027]
Therefore, in the portions 43, 45 , and 47 where the electrodes face each other, the liquid crystal is vertically aligned as in the example of FIG. On the other hand, in the portions 44 and 46 where the electrodes do not face each other, the liquid crystal is also affected by the electric field between the adjacent electrodes and is oriented in that direction. Therefore, alignment can be performed in a specific direction without providing an alignment film.
[0028]
According to the above-described method, the entire liquid crystal material is polymerized by heat, ultraviolet light or the like in a state where the distribution is imparted to the liquid crystal material, whereby the liquid crystal material can be solidified while the distribution of the alignment is fixed.
[0029]
The polymer liquid crystal of the present invention may be a polymer formed from such an unpolymerized liquid crystal material (liquid crystal monomer) as long as the unpolymerized liquid crystal material exhibits liquid crystallinity. What is necessary is just to show a refractive index anisotropy. A polymer having a large refractive index anisotropy is preferable, and a polymer having a refractive index anisotropy of 0.1 or more is preferable. Therefore, the polymer liquid crystal itself does not need to exhibit liquid crystallinity.
[0030]
The polymer liquid crystal is preferably produced by polymerizing a liquid crystalline monomer by light or heat. Particularly, a liquid crystal monomer which can be polymerized by ultraviolet light or visible light is preferable because a polymer liquid crystal can be produced on site (directly on a substrate) by a photolithography process.
[0031]
The liquid crystal monomer refers to a monomer that exhibits liquid crystallinity at room temperature or at the time of photopolymerization. The term “liquid crystallinity” refers to a known liquid crystal phase such as nematic, smectic, or cholesteric. However, a cholesteric having a short helical pitch is inappropriate for the present invention.
[0032]
The liquid crystal monomer is preferably selected from esters such as acrylic acid and methacrylic acid. It is preferred that the alcohol residue forming the ester contains one or more, particularly two or three phenyl groups. Further, one cyclohexyl group may be contained in the alcohol residue forming the ester. The liquid crystalline monomer can be used as a mixture of two or more components in order to widen the temperature range in which the liquid crystal can exist as liquid crystals.
[0033]
In embodiments of the present invention, each electrode is provided on the two transparent substrates, one of which is periodically formed Rukoto two electrodes are preferred. With such a configuration, the alignment state of the liquid crystal material when an electric field is applied can be made different between a portion corresponding to the periodically formed divided electrode and a portion where the divided electrode is not formed. The diffraction grating can be easily formed by an electric field.
[0034]
As in the example of the above-mentioned figure, when both of the two electrodes are formed periodically and the two electrodes are symmetric between the two transparent substrates, the optically anisotropic diffraction grating is emitted. Both ± first-order diffracted lights can have substantially the same diffraction efficiency.
[0035]
Further, in another aspect of the present invention, both of the two electrodes is formed periodically, the said two electrodes between two transparent substrates Ru is asymmetrical. Specifically, for example, when viewed in the example of FIG. 4, the electrode 32A protrudes to the electrode 32B only on the right side with respect to the electrode 31A, and the width of the electrode 32A is wider than the width of the electrode 31A.
[0036]
By doing so, in the state where the polymer liquid crystal cell is formed, the liquid crystal becomes asymmetric with respect to a center plane located at the center between the two transparent substrates and parallel to the two transparent substrates. With this configuration, the two electrodes each of the divided electrodes, will be the location及beauty atmosphere of the facing in a different state, when viewed in a pair of upper and lower divided electrodes, orientation by split electrodes of the liquid crystal polymer The portion may be asymmetrical. Therefore, an optically anisotropic diffraction grating having high diffraction efficiency of any of the ± 1st-order diffracted lights can be easily formed by an electric field.
[0037]
In the present invention, as described above, the alignment film is provided on the surface of at least one of the transparent substrates in contact with the liquid crystal, and at least one of the alignment films periodically includes an alignment film having a different alignment force. sell. Even when such a divided alignment film in which the alignment force is periodically changed, distribution can be imparted to the alignment state of the polymer liquid crystal. Furthermore, the alignment state of the polymer liquid crystal in the direction of the period may be left-right asymmetric. Therefore, it is possible to easily form an optically anisotropic diffraction grating having a partially high diffraction efficiency among the ± 1st-order diffracted lights.
[0038]
Further, both alignment films of the two transparent substrates can be formed as divided alignment films. Therefore, in a state where the polymer liquid crystal cell is formed, the alignment state can be asymmetric with respect to a center plane located at the center between the two transparent substrates and parallel to the two transparent substrates. With such a configuration, when viewed from a pair of upper and lower divided alignment films, each of the divided alignment films of the two alignment films can make the alignment portion of the polymer liquid crystal divided alignment film asymmetric. Therefore, an optically anisotropic diffraction grating having high diffraction efficiency of any of the ± 1st-order diffracted lights can be easily formed by the alignment film.
[0039]
Further, in yet another aspect of the present invention, by arranging a mask formed with periodically shielding pattern on the optically anisotropic diffraction element, while applying an electric field to the two transparent substrates electrodes entirely, or The liquid crystal is hardened only in the portion exposed to light by light exposure without applying an electric field, and then the application state of the electric field is changed to harden the remaining portion, and the orientation direction of the polymer liquid crystal is periodically changed. An optically anisotropic diffraction element can also be formed.
[0040]
Further, using such a mask, electrodes may be arranged outside the substrate to apply an electric field, or a magnetic field may be applied from outside the substrate to form a periodic orientation.
[0041]
The optical head device of the present invention optically functions as follows. The optically anisotropic diffraction grating is described as using an ultraviolet-curable unpolymerized nematic liquid crystal having a positive dielectric anisotropy, which is cured as shown in FIG. 2 by ultraviolet curing.
[0042]
The light emitted from the light source is reflected by the optical recording medium, the optical path is changed by the beam splitter, and is incident on the optically anisotropic diffraction grating. At this time, for the S-polarized light (polarized light component perpendicular to the paper surface of FIG. 2), the optically anisotropic diffraction grating has a horizontally aligned portion and a vertically aligned portion optically uniform. Rate. Therefore, the light is transmitted without any change.
[0043]
On the other hand, the P-polarized light (polarized light component parallel to the paper surface of FIG. 2) has a different refractive index between the vertically aligned part and the horizontally aligned part. That is, the vertical alignment portion has an ordinary light refractive index, and the horizontal alignment portion has an extraordinary light refractive index. For this reason, it is recognized as a diffraction grating, and theoretically ＋１40% of + 1st order light and 40% of −1st order diffracted light are obtained, and the transmitted light can be reduced to zero theoretically. The rest is higher-order light.
[0044]
Therefore, for example, by measuring the relative intensity ratio of the transmitted light amount and the ± first-order diffracted light amount, the polarization state of the reflected light from the optical recording medium and its change can be read.
[0045]
In the optically anisotropic diffraction grating of this example, the width of the electrodes was made the same between the two substrates, but the two periodic electrodes provided on the two transparent substrates were replaced by their positions and / or sizes. Is asymmetric, a polymer liquid crystal corresponding to an electrode portion and oriented in a specific direction by an electric field can be formed asymmetrically. Therefore, an optically anisotropic diffraction grating having high diffraction efficiency for any of the ± 1st-order diffracted lights can be obtained.
[0046]
The width W ₁ of the electrode may be about approximately half the electrode pitch, Ru is approximately 20% to 80% if necessary. The pitch W ₃ of the electrode may be used as appropriate optimized in the range of about 5 to 50 [mu] m.
[0047]
【Example】
[ Example 1 ] is a reference example, and [ Example 2 ] is an example.
[Example 1]
A solid transparent electrode of ITO is formed on one surface of a glass substrate having a thickness of 1 mm and 120 mm × 120 mm square, and the ITO transparent electrode is formed by photolithography and dry etching to have a width W ₁ = 10 μm and a pitch W ₂ = Patterning was performed in a stripe shape of 20 μm. Note that an antireflection film was formed on the opposite surface by an evaporation method.
[0048]
Thereafter, a polyimide film having a thickness of about 60 nm was formed by spin coating, and a horizontal alignment treatment was performed by rubbing. The two glass substrates thus formed are arranged so that the surfaces on which the transparent electrodes are formed face each other, the periphery is sealed with a sealing material, and an empty cell having a cell gap of 5 μm is provided. Formed.
[0049]
In the empty cell, p- [4-ω-acryloyloxyalkyloxy) phenylcarbonyloxy] benzonitrile and p- (4-ω-acryloyloxyalkyloxy) benzoic acid-p′-n-alkyloxyphenyl ester are mainly used. A composition (operating as a nematic liquid crystal having positive dielectric anisotropy) in which 1% of benzoin isopropyl ether was added as a photopolymerization initiator to a liquid crystal monomer as a component was injected.
[0050]
Thereafter, an electric field of 5 V was applied between the electrodes of the two substrates, and the liquid crystal composition in a portion where the electrodes were opposed was vertically oriented. Thereafter, ultraviolet light having a wavelength of 360 nm is irradiated on the whole, and the whole is polymerized and cured while maintaining the above-mentioned orientation, thereby fixing the orientation.
[0051]
The optically anisotropic diffraction grating thus formed has a refractive index of 1.52 (ordinary refractive index) at the electric field application portion and a non-application portion at the non-application portion for S-polarized light (light polarized perpendicular to the plane of FIG. 2). With respect to a refractive index of 1.53 (ordinary refractive index) and P-polarized light (light polarized in a direction parallel to the paper surface of FIG. 2), the refractive index is 1.54 (ordinary refractive index) in the electric field application section and non-application section A refractive index of 1.66 (an extraordinary light refractive index) was obtained, and a refractive index difference of about 0.12 was obtained.
[0052]
In the optical head device having the configuration shown in FIG. 1 using this optically anisotropic diffraction grating, when a laser light source having a wavelength of 678 nm is used as a light source, the light transmittance for incident S-polarized light is 85%, and the light diffraction efficiency is ±. It was 0.5% for the first order. The light transmittance with respect to the P-polarized wave incident light was 3.4%, and the light diffraction rates were 34% for the + 1st order and 32% for the -1st order.
[0053]
[Example 2]
A diffraction element was manufactured in the same manner as in Example 1 except that the configuration of the transparent electrode of Example 1 was changed as follows.
[0054]
The transparent electrode of one glass substrate was patterned in a stripe shape having a width W ₁ = 10 μm and a pitch W ₂ = 20 μm. The transparent electrode of the other glass substrate was patterned in a stripe shape having a width W ₁ = 5 μm and a pitch W ₂ = 20 μm. An optical head device was formed by forming an optically anisotropic diffraction grating in the same manner as in Example 1 except that an empty cell was formed using such a substrate. In this optical head device, the efficiency of the diffracted light was different between the + 1st-order diffracted light and the -1st-order diffracted light.
[0055]
【The invention's effect】
According to the present invention, an optical head device that can be mass-produced, has high light use efficiency, and has high reliability can be easily obtained. In addition, by appropriately selecting a liquid crystal material, a desired refractive index can be freely set in various ways, and a shape and a pitch of a portion having different orientation can be freely selected, so that it is easy to manufacture an optical head device having desired characteristics and arrangement. .
[0056]
Furthermore, by making the alignment state of the liquid crystal asymmetric between the two transparent substrates, an asymmetric optically anisotropic diffraction grating can be easily formed. It can be easily manufactured.
[0057]
The present invention can be applied to various applications within a range that does not impair the effects of the present invention.
[Brief description of the drawings]
FIG. 1 is a schematic view of an optical head device according to the present invention.
FIG. 2 is a cross-sectional view of an optically anisotropic diffraction element used in the optical head device of the present invention.
FIG. 3 is a plan view of an example of a periodic electrode of the optically anisotropic diffraction element used in the present invention.
FIG. 4 is a schematic view showing an alignment state of a liquid crystal in a state where an electric field is applied to an optically anisotropic diffraction element used in the present invention.
FIG. 5 is a schematic diagram showing an alignment state of a liquid crystal in a state where an electric field is applied to an optically anisotropic diffraction element used in the present invention.
[Explanation of symbols]
1: light source 2: beam splitter 3: condensing lens 4: optical recording medium 5: optically anisotropic diffraction gratings 6A, 6B, 6C: detection unit

Claims

An optical head device for reading information and / or writing information by irradiating light from a light source onto an optical recording medium, wherein light from the light source is transmitted between the light source and the optical recording medium and A beam splitter that changes the optical path of the light so that the reflected light does not return to the light source is arranged. The reflected light whose path has been changed passes through the optically anisotropic diffraction element, and is divided into a plurality of diffracted lights. In the optical head device for detecting light by using, the optically anisotropic diffraction element includes a polymer liquid crystal having optical anisotropy and having a periodically changing orientation direction sandwiched between two transparent substrates. An electrode is periodically formed on each of the transparent substrates, the width of the electrode on one transparent substrate is smaller than the width of the electrode on the other transparent substrate, and the two electrodes are arranged asymmetrically. , Curing the liquid crystal while applying an electric field to those electrodes By an optical head device, wherein a periodic variation in the orientation direction of the polymer liquid crystal is formed.

The width of periodically formed electrode of the can, according to claim 1 Symbol placement of the optical head apparatus 20 to 80% of the electrode pitch.

The electrodes of at least one of the two transparent substrates of the optically anisotropic diffraction element are patterned, and when an electric field is applied to the electrodes, the liquid crystal forms a natural arrangement in a portion where the electrodes do not face each other. In the portion where the electrodes face each other, forced alignment is performed, and the liquid crystal is cured in this state, whereby the optically anisotropic diffraction element in which the alignment direction of the polymer liquid crystal is periodically changed is formed. 3. The optical head device according to claim 1, wherein

An optical head device for reading information and / or writing information by irradiating light from a light source onto an optical recording medium, wherein light from the light source is transmitted between the light source and the optical recording medium and A beam splitter that changes the optical path of the light so that the reflected light does not return to the light source is arranged. The reflected light whose path has been changed passes through the optically anisotropic diffraction element, and is divided into a plurality of diffracted lights. In the optical head device for detecting light by using, the optically anisotropic diffraction element has a polymer liquid crystal having optical anisotropy and having a periodically changing orientation direction sandwiched between two transparent substrates. An electrode is provided on each of the two transparent substrates, and is periodically placed on the diffraction element while applying or not applying an electric field to the entire surface of the electrodes on the two transparent substrates. Perform light exposure using a mask with a light-blocking pattern. Anisotropic diffractive element in which the orientation direction of the polymer liquid crystal changes periodically by curing the liquid crystal only in the area where the light hits and then curing the remaining part by changing the applied state of the electric field. An optical head device comprising:

The optical head device according to claim 4 , wherein at least one of the two electrodes provided on each of the two transparent substrates is formed periodically.